1. Field of the Invention
The present invention relates to a wireless communication system comprising a plurality of base stations installed within a wireless communication network and mobile stations which perform wireless communication with such base stations and, more particularly, to a wireless communication system which may be advantageously used for the construction of a wireless LAN system. Such wireless LAN systems are spreading rapidly in such places as offices and hot-spots.
2. Description of the Related Art
In recent years, wireless LAN systems using wireless communication systems have been spreading rapidly in offices of various companies as corporate LANs and in hot-spots set up in shops or in public facilities.
Usually, a wireless LAN system comprises, as described in Japanese Unexamined Patent Publication No. 2001-16149, a communication network such as the Internet or an intranet, a host computer connected to the communication network, a mobile station (mobile node) for a user to access the host computer, and a base station as an access point which performs wireless communication with the mobile station and enables the mobile station to access the host computer by delivering transmit data placed on the communication network to the mobile station or transmitting data received from the mobile station onto the communication network.
In such a wireless LAN system, a personal computer or a personal digital assistant (PDA) is often used as the mobile station, and the user at the mobile station performs wireless communication with the base station by using a wireless LAN function built into the personal computer or personal digital assistant or, if such a wireless LAN function is not built in, then by providing the necessary wireless LAN function by installing a wireless LAN card in an expansion slot of the mobile station.
In many cases, conventionally-used wireless LAN functions and wireless LAN cards have built-in antennas whose directivity is almost omnidirectional. Such omnidirectional antennas permit communications in all directions, but are not suitable for achieving selective communication functions.
Accordingly, when a plurality of base stations are installed one adjacent to another to cover mobile stations located within a given area, the area is divided into smaller areas each one covered by a base station so as to prevent interference with radio waves radiated from adjacent base stations.
Then, when there arises a need for the mobile station to switch to another base station as the mobile station moves around, the mobile station receives identification information called a beacon frame that every base station transmits, and switches to the new serving base station based on the base station identifier (for example, SSID: Service Set Identifier) included in the received beacon frame. With such a method, the mobile station can switch its serving base station to the nearest base station as it moves around.